[meteorite-list] Self-Destructing Comet to Flash Close By (Schwassmann-Wachmann 3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Apr 24 20:45:14 2006
Message-ID: <200604241644.JAA00869_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9035-selfdestructing-comet-to-flash-close-by.html

Self-destructing comet to flash close by
Kimm Groshong
New Scientist
April 24, 2006

Astronomers will soon be treated to a close-up celestial show, with a
fragmenting comet streaming across the sky in more than 30 chunks. Comet
73P/Schwassmann-Wachmann 3 has been breaking up since 1995, but between
12 and 14 May will come closer to the Earth than any comet since 1983.

Fortunately no threat is posed to Earth since, even at its closest, the
nearest of the pieces will be twenty times more distant than the Moon.

But astronomers around the world will take advantage of the relatively
close pass. The Hubble and Spitzer Space Telescopes will join with
dozens of ground-based facilities for the observation campaign. And
scientists expect the brightest comet pieces to be visible through
binoculars and possibly, faintly, to the naked eye.

By studying the materials exposed by the break-up, astronomers hope to
learn more about the interior of comets, where pristine ingredients from
the solar system's earliest days are thought to be preserved. For that
reason, some astronomers view the event as a free version of NASA's
comet-busting Deep Impact mission.
          
Bursts of brightness

They also hope to gain insight about how and why such comets become
fragmented. For largely unknown reasons, Comet 73P broke into as many as
five pieces in 1995. Gravitational forces are known to tear apart some
comets, such as the famous Comet Shoemaker-Levy 9 which ended its
existence with a spectacular crash into Jupiter. But Comet 73P
follows a 5.4-year solar orbit that does not bring it close enough to
Jupiter or the Sun for gravitational tugs to be the culprit.

The initial trigger that set Comet 73P's fragmentation process in action
in 1995 is still a matter of speculation. "We suspect that rotation may
contribute, but it may not be rotation alone. It could be that thermal
stresses are also contributing," says Zdenek Sekanina, a cometary
physicist at the Jet Propulsion Laboratory in California, US.

A clue may come from the chronology of observations in 1995. Astronomers
observed two sudden bursts of brightness from the comet in September and
November. Then in December, observers caught the first glimpse of Comet
73P in its piecemeal form.

Such flare-ups are thought to be caused by the sublimation of newly
exposed ice from the interior of the comet. Splitting comets have often
been detected because of these flare-ups. But it remains unclear whether
the outbursts actually cause comets to split or are simply an
after-effect of a split that occurred through other processes.

Missing pieces

In any case, comet 73P is still breaking up. The remnant of the parent
comet is known as component C and other pieces have been given letter
names progressing through the alphabet. And as of this week, astronomers
have reached named 37 fragments.

Some of the pieces have survived two orbits around the Sun since the
original divide in 1995. But others have been lost. For example,
components A, D, E and F have not been spotted this year. They may have
broken into pieces too small to see or they may simply be inactive, and
therefore invisible. But at least 33 mini-comets may be able to put on a
light show when passing Earth.

Some fragmenting comets, such as Comet Biela in the 19th century, have
produced meteor showers when flying by our planet. But Paul Wiegert, of
the University of Western Ontario, does not expect a meteor shower in
May, based on the nature of the 1995 break up.

He says the comet's cloud of debris is not likely to reach Earth until
2022. "But things have changed a little bit because the comet is
continuing to break up," he adds. "So it's a little bit hard to say
whether we will see meteor showers this year."
Received on Mon 24 Apr 2006 12:44:31 PM PDT